As an example of a propulsion device of a ship, a system of single-engine single-axis (one main engine and one propeller) and a system of twin-engine twin-axis (two main engines and two propellers) are known. As the propulsion device of a general commercial ship, the single-engine single-axis system and the twin-engine twin-axis system are often adopted. The ship which adopts the former is called a single-screw ship, and the ship which adopts the latter is called a twin-screw ship.
Also, in recent years, as the ship becomes larger in size, problems are caused such as the lowering of propulsive efficiency in accompaniment with increase of a load to a screw propeller, and the increase of hull vibration and the occurrence of erosion in accompaniment with extension of a cavitation range in the single-screw ship. It is known that these problems can be solved by the twin-screw ship. In the twin-screw ship, loading one propeller is reduced to improve the propeller efficiency and the occurrence range of the cavitation can be narrowed.
As an example that two screw propellers are provided at the stern of a ship, an overlapping propeller (OLP) type, an interlock propeller type, a two-propeller parallel arrangement type, and so on are known. In the OLP type, two propellers are arranged to be displaced in a forward or backward direction, such that the two propellers are overlap each other when viewed from the stern. The propulsion efficiency can be improved by 5-10% in the OLP type of ship, compared with that of the single-screw ship. Also, in the interlock propeller type of ship, the propellers are arranged such that each wing of one screw propeller appears between the wings of the other propeller. In the two-propeller parallel arrangement type of ship, the two propellers are arranged symmetrically in parallel to each other in a longitudinal direction of the ship.
Here, when two screw propellers are arranged in the stern structure of a single-screw ship (having a skeg type of stern in which a stern central portion is made thin to bring the propellers close to each other), it is desirable from the viewpoint of a slow water flow near the hull centerline and longitudinal vortices such as bilge vortices that the propellers are arranged in the neighborhood of the hull centerline. In the propeller position of a usual single-screw ship, the longitudinal vortices of a slow water flow, which are such as a pair of the bilge vortices symmetrical with respect to the hull centerline and rotating into an inboard direction, are generated in the stern. Because the propeller is designed to have a high efficiency in the slow flow, the propulsion efficiency can be improved by rotating the propeller near the longitudinal vortices and collecting the slow flow and the longitudinal vortices in the neighborhood of the hull centerline. In case of the OLP type of ship, the outboard direction is often adopted as the rotation direction of the propeller, in order to collect the longitudinal vortices near the hull center efficiently for improvement of propulsion performance.
For example, in Patent Literature 1 (WO2006/095774), a technique is disclosed in which the propeller loading and the generation cavitation can be reduced when using the OLP structure for the stern portion of a single-screw ship.